Accumulation of damaged or denatured proteins is associated with cataract. Efficient removal of damaged proteins or renaturation of the denatured proteins is crucial for the maintenance of lens transparency. The ubiquitin-proteasome pathway (UPP) is a quality control mechanism in this process. As in most types of cells, lens cells, including the fibers have a fully functional UPP and the UPP is involved in degradation of oxidized proteins in lens cells. The UPP also has a role in preventing heat-induced precipitation of lens proteins. The regulatory PA700 complex of the 26S proteasome facilitates the reactivation of denatured enzymes. The long term goal of this project is to test the hypothesis that the UPP is a master quality control mechanism which recognizes and degrades or repairs the damaged or denatured proteins. The specific aims of this proposed grant period are to determine the role of the UPP in selective degradation of oxidized, glycated and truncated lens proteins in a cell-free system and in intact lens cells. The mechanisms which underlie the recognition of damaged proteins by the UPP will be investigated. In addition, the molecular chaperone function of the UPP in renaturation of denatured proteins will be evaluated in lens cells. Oxidized, glycated, and truncated proteins are commonly observed in cataractous or aging lenses and their accumulation is detrimental to the transparency of the lens. Demonstration of the mechanism which selectively degrades these damaged proteins will provide guidance for designing strategies to remove the damaged proteins from the lens cells and prevent or delay cataractogenesis. Denatured proteins are unstable and tend to aggregate and cause light scattering. Chaperone-mediated stabilization and renaturation of lens proteins are important for maintaining the solubility of lens proteins and lens transparency. Therefore, characterization of the ubiquitin-dependent chaperone activity in lens cells will shed light on the mechanism for lens transparency and cataractogenesis. Identification of the molecules which are responsibe for the recognition of abnormal proteins will build the foundation for future studies which will aim to extend the functional life of the lens by targeted expression of these key molecules via gene therapy techniques.